1. Field of the Invention
The field of the invention relates generally to split ignition timing for idle speed control of an engine.
2. Background of the Invention
Engine idle speed control is typically accomplished in stoichiometric engines by adjusting airflow to control engine speed to a desired engine speed. However, since airflow control may be slow due to manifold volume dynamics, ignition timing adjustment is also used. To allow for ignition timing adjustment to increase engine torque, nominal operation of idle speed control requires some basic amount of ignition timing retard. This ignition timing retard is adjusted based on engine speed error to provide fast engine torque control. In addition this ignition timing retard provides some heat to improve catalyst light-off.
The inventors herein have recognized a problem with the above approach. In particular, the amount of ignition timing retard is limited since too much retard may affect engine combustion. In other words, large amounts of retard may cause vibration, which can reduce customer satisfaction. That is, the amount of ignition timing retard (and therefore heat to the catalyst) is limited. Further, since this retard amount is limited, the range of authority for increasing torque via advancing the ignition timing is also limited and large torque demands due to the A/C compressor or power steering pump for example, can cause engine speed variation during idle.
Also, when using an electronic throttle in the above approach, engine idle speed control can be degraded since at low throttle positions (used during idle speed control), a small change in throttle position can have a relatively large change in engine airflow.
Various of the above disadvantages are overcome by operation according to a method for controlling an engine have at least first and second groups of cylinders, the engine coupled to an emission control device. The method comprises (i) operating the first group of cylinders at a first ignition timing, (ii) operating the second group of cylinders at a second ignition timing more retarded than the first group, (iii) determining a signal based on a desired and actual speed, and (iv) adjusting the first ignition timing based on the signal.
By operating one group of cylinders with more ignition timing retard than others, it is possible to operate the engine at a higher load, thus providing more heat flux to the engine exhaust. Further, the cylinders with less ignition timing retard can provide most of the engine output torque and thereby control engine idle speed, while the other cylinders with more retard generate even greater heat.
Further, since the engine cylinders are operating at a higher load, the amount of ignition timing retard that can be tolerated, while still providing stable combustion, can be increased. This results in even more heat generation.
Further, still, since the engine is operating at a higher load, better airflow control is achieved since the change in throttle position to change in airflow slope is decreased.
Finally, by maintaining idle speed control via adjustment of the ignition timing of the first cylinder group, it is possible to provide large amounts of heat while maintaining idle speed at a desired level.
In one aspect of the present invention, both airflow and ignition timing are adjusted to maintain engine idle speed control. In still another, engine output is first increased by advancing ignition timing of the first cylinder group, and then advancing ignition timing of the second cylinder group. In this way, a very large range of engine torque authority is achieved via adjustment of both the first and second ignition timing.
In another aspect of the present invention, the cylinder groups are operated at different ignition timing values after an engine is started with all of the cylinders at substantially the same ignition timing.
Note that there are many ways to indicate engine starting. For example, any of the following conditions can be used: engine speed greater than some level, engine firing synchronous or after a certain time, or after starter cranking, or after a predetermined time.
Also note that there are various methods for adjusting ignition timing to maintain desired engine speed. For example, a PID (proportional, integral, derivative) controller can be used. Alternatively, a feed-forward plus feed-back approach can be adopted.